I have to say that I struggle a bit with the theory of the 'unselective uterus'. For one, I think that it is normal and natural that a uterus would implant any fertilised egg, since this is what it is meant to do. At such an early stage I doubt that a fertilised egg could send out any signals that indicate it's not a 'good one' - there just isn't enough there to produce enough of any type of signal, iykwim.
I know that we all (myself included!) have at one point raged against our bodies and that they can't seem to hold on to a baby, but I sometimes think that we forget that in most cases our bodies have done exactly what they are supposed to do - that is, to stop a non-viable pregnancy. It is horrible and soul-destroying, but it has to happen! Just imagine if it wouldn't...
We also often forget that most miscarriages do happen for a reason, ie. there is something specific and definitely wrong with the fetus, which is not compatible with life - however, in most cases, we will never know why the miscarriage happened. Pregnancy science is one of the most undeveloped medical fields, so much is just unknown.
One of the main issues with mc is that it really messes with our relationship with our own bodies...